1. Field of Invention
This invention relates generally to vessel mooring systems. In particular, the invention relates to improved disconnectable mooring systems by which a mooring system supported by a buoyant assembly may be quickly connected and disconnected from a turret of a vessel.
2. Description of the Prior Art
With the occurrence of offshore sub sea production wells came the need for floating production vessels to accept the product of such wells. Certain offshore oil fields are in waters in which fierce storms occur or in which ice floes are present. For such environments there has developed disconnectable mooring systems so that a mooring element may be permanently placed at the field and connected and disconnected to the production vessel. When dangerous weather conditions are forecasted, the vessel disconnects from the mooring system and sails to safe harbor to wait out the storm or ice floe. The mooring system remains on location. When storm conditions pass, the vessel returns to the field, reconnects to the mooring system, and production resumes.
One such system is illustrated in U.S. Pat. No. 4,650,431 to Kentosh. Such patent issued Mar. 17, 1987 from a continuation in part application dated Sep. 15, 1980. The Kentosh patent illustrates a turret rotatably mounted to a ship. A mooring buoy may be connected and disconnected from the bottom of the turret. The mooring buoy is fixed to the sea floor by means of a plurality of anchors connected to the mooring element by catenary chains. One or more risers run from production wells on the sea floor to the mooring buoy where they are connected to conduits in the turret and ultimately to a product swivel to conduits running to holds in the vessel. The vessel includes bearings which provide support to the turret while allowing the vessel to weathervane about such turret under forces of wind, waves and currents.
The mooring system described in the Kentosh patent is supported by a buoy that can be mechanically connected to a turret. The level of buoyancy of such buoy and the weight and design of catenary chains and anchor system are coordinated such that when the vessel disconnects from the buoy, the weight of the chains cause the buoy, though buoyant, to sink. As the chains lay down on the sea floor with the sinking of the buoy, less and less downward force is applied to it the deeper the buoy sinks. An equilibrium point is reached where the upward force due to the buoyancy balances the downward force of the chains. An equilibrium depth of at least five meters below average sea level is described to avoid damage from ice packs and to reduce wave action forces. A marker buoy is attached via a line to the mooring element.
U.S. Pat. No. 4,604,961 issued Aug. 12, 1986 to Ortloff et al (Ortloff) based on an application filed Jun. 11, 1984. A well or moon pool is provided between the bow and stern of the production vessel. A turret is rotatably secured in the well at a position at the bottom of the vessel. A mooring system may be connected or disconnected to such turret. Once the mooring system is connected to the turret, the vessel is free to weathervane about the turret by means of anchors and catenary chains that are secured to the sea floor. The buoy supporting the mooring system is stored beneath the sea surface when the vessel disconnects from the mooring element. Like in the Kentosh system, the buoyancy of the Ortloff support buoy is designed such that it reaches equilibrium against the decreasing downward forces of the catenary chains with the sinking of the mooring element.
A published paper, OTC 6251, titled Innovative Disconnectable Mooring System for Floating Production System of HZ-21-1 Oil Field at Huiyhon, South China Sea by G. O'Nion, et al., presented at the 22nd Annual Offshore Technology Conference, May 7-10, 1990 describes a disconnectable buoyant turret mooring system to moor a tanker floating production system.
The described system includes a turret located in the forepeak structure of a tanker floating production system. Eight equally spaced catenary anchor legs are connected to the turret by means of a submerged buoy. The buoy is connected to the turret structure by means of a collet type structural connector. During connection operations of the buoy to the turret, a wire rope connected to the buoy is hauled in on a drum winch located on the deck of the vessel.
The turret of the O'Nion system is supported to the vessel by a three-race roller bearing, located just above the keel structure of the vessel. Such bearing allows the vessel to weathervane about the turret fixed to the sea floor by means of a buoy/catenary line/anchor system.
Mooring loads between the vessel and the buoy/turret are transmitted via the three-race roller bearing. Bending moment loading on the turret occurs because the supporting three-race roller bearing is axially separated from the connector which secures the turret to the mooring buoy.
The O'Nion system includes a re-connection wire rope which dangles below from an axial passage of the buoy. A floating mooring line extends from the surface of the sea to the top end of the re-connection wire end of the buoy. The floating synthetic mooring line is used to draw the vessel to the mooring buoy by heaving in the mooring line with a winch on the deck of the vessel. The re-connection wire rope is ultimately heaved in from beneath the mooring buoy as it is slowly drawn through the axial passage through the buoy and up into the turret. Lifting of the buoy is achieved by heaving in the reconnection wire rope.
The buoy is guided into registration with the turret by a guide pin facing downward at the bottom of the turret. With the buoy held firmly under the vessel by the upward tension in the wire rope, the turret is rotated with respect to the vessel until the buoy and turret have their respective riser tubes aligned. Once alignment is confirmed, either directly visually with a diver or indirectly visually by means of video equipment, the guide pin is extended downwardly into a hole in the top deck of the buoy. The connector between the turret and the buoy is then engaged. The risers extending to the buoy are then connected to risers of the turret.
While the O'Nion system offers advantages over disconnectable mooring systems which preceded it, there are a number of disadvantages inherent in its design.
First, the single bearing which supports the turret near the hydraulic connector at the bottom of the turret is submerged and must be protected against ingress of sea water and is subject to relatively large dynamic moment loads, axial loads and radial loads.
Second, the hydraulic connection between the bottom of the turret and the top of the buoy must for practical reasons be of relatively small dimensions compared to the mass of the attached mooring buoy and anchor leg system. The components of the connector will consequently be subject to relatively large stress variations and also to stress reversals, due to the dynamic moment loads that will be acting directly on the connector during rough weather conditions. Such stress variations and reversals greatly increase the probability of fatigue failure of the connection. The hydraulic connection does not appear to have a mechanism to establish pre-load tension between the hydraulic connector of the turret and a connector hub atop the buoy. Furthermore, there appears to be no means to achieve automatic alignment of the turret with the buoy when the hydraulic connector connects to the connector hub.
Third, with the O'Nion system, it appears difficult to obtain the required rotational alignment between the turret and the buoy during the connection operations. There will be relatively high friction resistance to rotational movements between the turret and the buoy during the final stages of the pull-up operation. The reaction to rotational movement of the buoy afforded by the anchor chains will be too compliant to enable the final adjustment to be made within the required tolerance. Furthermore, the O'Nion system seems to require direct observation of an alignment pin on the turret with an alignment hole on top of the buoy.
Fourth, the O'Nion system does not appear to provide a way to test the mating and connection between the bottom of the turret and the top of the buoy prior to deployment of the vessel and mooring system in the sea.
The O'Nion system also does not provide an arrangement for storage and tangle-free deployment of a soft messenger line connected to the buoy mooring link during disconnection of the mooring buoy from the turret.